kjdragonkiller: June 2023 - got this working finally after 5 months of it being broken.
Steps on windows 10
1- Log out and close GOG galaxy! Important!!!
2- Go to C:\Users\<Insert YOUR USERNAME>\AppData\Local\GOG.com\Galaxy\plugins\installed\steam_ca27391f-2675-49b1-92c0-896d43afa4f8
(you will need to view hidden files to get access to this folder sometimes)
3- delete all steam files in there. Don't delete the folder.
4- Go to github link in this forum thread github.com/ABaumher/galaxy-integration-steam/releases/tag/Beta
5- Download zip and extract files.
6- Then paste files into that folder above.
7- login to gog galaxy
8- go to settings and integrations
9- insert your steam user id and password
10- complete authentication verification on your mobile app...
and boom you now have your whole steam lib in there... mine went from 500 to 900 !
Works great for Windows 11 as well. Both the 1.2.1 beta & the 1.0.8 work just fine. The github page also has these steps in their install instructions.